Large quantities of maleic anhydride are produced each year throughout the world, since maleic anhydride can be employed as a versatile intermediate for chemical synthesis and is often used in the production of alkyl resins. In the past, maleic anhydride was produced commercially by either the catalytic oxidation of benzene or the catalytic oxidation of butenes. An important method formerly employed commercially for the manufacture of maleic anhydride consisted of air oxidation of benzene in the presence of a heavy metal oxide catalyst. Since benzene fumes are toxic and since the use of butane is more economical than benzene, the trend has been to minimize or eliminate the use of benzene as a feedstock and to oxidize either butenes or butane to maleic anhydride.
A typical catalyst that can be used for the oxidation of butane to maleic anhydride is a catalyst comprising the oxides of vanadium and phosphorus. For example, in U.S. Pat. No. 3,293,268, Bergman, et al., teach that the oxidation of n-butane or other saturated aliphatic hydrocarbons having from 4 to 10 carbon atoms can be performed under controlled temperature conditions in the presence of a specified class of phosphorus-vanadium-oxygen-containing complex catalysts. While such a catalyst is capable of oxidizing butane, it does not give sufficiently high yields.
Attempts have been made to overcome this disadvantage by employing various activators, stabilizers, and promoters in the catalyst in order to improve the yields of maleic anhydride. For example, in U.S. Pat. No. 3,862,146, Boghosian discloses the oxidation of n-butane to maleic anhydride in the presence of a phosphorus-vanadium-oxygen complex catalyst containing zinc, bismuth, copper, or lithium as an activator. Raffelson, et al., teach the use of such catalyst components in U.S. Pat. No. 3,867,411; Young, et al., in U.S. Pat. No. 3,888,886; and Higgins, et al., in U.S. Pat. No. 4,147,661. Unfortunately, the phosphorus-vanadium-metal-promoted catalysts tend to lose selectivily rather rapidly.
This necessitates the reactivation or regeneration of these catalysts. In U.S. Pat. Nos. 4,020,174; 4,094,816; and 4,089,807; the patentees teach that a vanadium-phosphorus-metal-promoted catalyst can be reactivated by the use of carbon tetrachloride. In U.S. Pat. Nos. 3,296,282 and 3,474,041, the patentees disclose the process of treating a vanadium-phosphorus oxidation catalyst with a phosphine, phosphite, or phosphonate by periodically or continuously passing such phosphorus compound to the reactor, either with or without interrupting the olefin feed flow that is being used to make the maleic anhydride. United Kingdom Patent Specification 1,464,198 discloses the reactivation or regeneration of certain vanadium-phosphorus-oxygen catalyst complexes promoted with zirconium, hafnium, chromium, iron, lanthanum, or cerium by having the catalyst contacted during vapor-phase oxidation with an alkyl ester of orthophosphoric acid having the formula (RO).sub.3 P.dbd.O, wherein R is hydrogen or a C.sub.1 to C.sub.4 alkyl radical, at least one R being a C.sub.1 to C.sub.4 alkyl radical.
In U.S. Pat. No. 4,701,433, Edwards discloses a process for the manufacture of maleic anhydride from butane in the presence of a vanadium-phosphorus-oxygen catalyst or a vanadium-phosphorus-oxygen-co-metal catalyst, wherein water and a phosphorus compound are added to the reaction system to reversibly deactivate a portion of the catalyst in the catalyst bed containing a reaction exotherm (hot spot) prior to the addition of the phosphorus compound, which addition of phosphorus compound moves the reaction exotherm downstream into the catalyst bed, and an improved catalyst bed is obtained when the partially deactivated catalyst in the original "hot spot" location reactivates to produce a more isothermal catalyst bed.
Although Edwards, et al. recognized the utility of a more isothermal catalyst bed to improve yield, Edwards accomplished a more isothermal catalyst bed temperature by shifting the location of the original "hot spot" to a new location and then reactivating the old location. Edwards teaches that improvement in yield can be obtained as long as there is sufficient catalyst bed into which the exotherm may migrate. Edwards failed to recognize that constant shifting of the exotherm is inherently unstable and not suitable for long-term operation.
U.S. Pat. No. 4,780,548, Edwards, et al. teaches a continuous process for the vapor-phase oxidation of a n-butane feedstock to form maleic anhydride in which n-butane is contacted in the presence of molecular oxygen or air at an hourly space velocity of about 100 to about 4000 cubic centimeters of feed per cubic centimeter of catalyst per hour with a vanadium-phosphorus-oxygen catalyst wherein the catalyst is regenerated continuously or batchwise by contacting it during the vapor phase oxidation with an alkyl ester of orthophosphoric acid having the formula (RO).sub.3 P.dbd.O where R is hydrogen or a C.sub.1 to C.sub.4 alkyl, at least one R being a C.sub.1 to C.sub.4 alkyl, wherein the amount of water added is about 1000 parts per million to about 40,000 parts per million by weight of the reactor feed gas stream and the amount of the alkyl ester added is about 0.1 parts per million to about 100,000 parts per million by weight of the reactor feed gas stream. The gaseous feed stream to the reactor will contain from about 0.2 to about 1.7 mole % of n-butane but about 0.8 to about 1.5 mole % of n-butane is satisfactory for optimum yield from the process of the invention. Edwards teaches that higher concentrations can be employed but explosive hazards may be encountered. Even though Edwards acknowledged that explosive mixtures could be employed, above 1.5-1.7 moles n-butanes, Edwards did not recognize that an isothermal reaction employing a higher concentration of n-butane would result in a yield continuously maintained at a high level for extended periods of time despite the possibility of explosive hazards.
Becker, et al., U.S. Pat. No. 4,795,818 disclosed a method for optimizing the yield of a vanadium-phosphorus catalyst during the oxidation of n-butane to maleic anhydride, wherein a volatile phosphorus compound is continuously added at a rate selected to maintain maximum yield while holding the operating temperature constant, the operating temperature being monitored preferably by the outlet gas temperature. Becker et al., indicate that the amount of phosphorus compound to be added should be sufficient to prevent decline in operating temperature. Becker, et al., fail to recognize the impact of the water added to the reaction and rely only on the addition of a volatile phosphorus compound to the process.
In U.S. Pat. No. 4,515,899, Click, et al., teach that the useful life of a vanadium-phosphorus-oxygen catalyst can be extended in fixed bed reactors by treatment with a phophorus compound followed by steam treatment and furnish data showing movement of the exotherm further into the catalyst bed.
In the partial oxidation of n-butane to maleic anhydride, the reaction is highly exothermic and a catalyst hot spot temperature can develop with potential of a runaway oxidation reaction and consequent complete loss of product yield. Such a development of a hot spot temperature is potentially extremely detrimental to the progress of the oxidation reaction. The hot spot temperature readily occurs in the oxidation and is quite sensitive to variations in the concentration of feed hydrocarbon. Small increases in the concentration of the hydrocarbon feed can result in large increases in hot spot temperature and a concurrent decrease in selectivity and yield. In addition, high hot spot temperatures can shorten the useful life of the catalyst being employed. It is therefore necessary to avoid the development of an excessively high hot spot temperature and to maintain an isothermal catalyst temperature range over the entire length of the reaction zone to obtain consistent high yield and lengthened catalyst life. Also, consistent high product yield requires a process with consistent process parameters.
It has now been found that the beneficial effect of adding a phosphorus compound in water, in certain ratios to each other, to control the reaction temperature profile in oxidation of n-butane to maleic anhydride can be obtained over the entire reaction zone of a fixed bed reactor. The beneficial effect occurs over the entire reaction zone including the so-called hot spot temperature zone but also a significant beneficial effect of the reaction temperature profile has been found to result despite operation in the flammability zone which exists wherein concentration of n-butane in the feed is about 1.7 mole %, or higher, and air is the source of oxygen. The required ratio of water to phosphorus in the phosphorus compound relates to the concentration of n-butane, and also to the reactor size and shape. An isothermal reaction zone temperature thereby results wherein the reaction zone temperature gradient is within a maximum range of about 45.degree. C. (80.degree. F.) with consequent increase in overall yield.
It has now been found that an isothermal reaction zone temperature wherein n-butane concentration in the feedstock is greater than 1.6 mole percent results in consistent high yield of maleic anhydride. The continuous addition of a phosphorus-water solution to the reaction in an essential ratio of phosphorus to water causes the effect of the addition to occur over the entire reaction zone of the reactor to decrease the so-called hot spot temperature and increase the over-all reaction temperature throughout the reaction zone to result in an isothermal reaction zone, which includes the so-called hot spot, wherein the temperature gradient in the reaction zone is within the range of about 45.degree. C. (80.degree. F.).